Which countries will be first able to fly after the covid 19 ?

I have never heard of a country that can fly, except Laputa in "The Castle in the Sky" by Studio Ghibli.
But in case you meant which countries will allow commercial passenger aircraft to fly again: It is still allowed in most places, just the demand is very low due to travel restrictions and lockdowns.

You can enter quite a few countries but you have to go into quarantine for 14 days and also with many places on lockdown its quite pointless in travelling.

It doesn't much matter which ones are first to let you take off; until there are places that let you land where you actually want to go, might as well stay home.

Some airlines can make you check that. So long as you know where you are headed. I believe, I was on emirates when I checked flights. They do however tell you to still check official websites as they are updated more regularly.

Probably major hubs like Singapore. Personally I would not trouble myself unless necessary considering that the end to this pandemic is nowhere in sight.